1. Field of the Invention
The invention pertains to the field of tensioners. More particularly, the invention pertains to locking devices for timing chain tensioners.
2. Description of Related Art
The timing system of an internal combustion engine may be controlled by a chain transmission, in which the chain is wound on two or more sprockets, one of which is a drive sprocket and takes its drive (even indirectly) from the drive shaft to transmit it, by means of the chain and of one or more driven sprockets, to one or more driven shafts.
Since for reasons of adjustment, wear of materials and/or of take-up of the slack, it is often necessary to compensate for a certain slack on the chain, shoe-type tensioning devices are known in the art. In these tensioners, a shoe is biased with an adjustable force against a strand of the chain, generally known as the “slack strand”.
The most frequently used tensioning devices are hydraulic or mechanical tensioners, in which a fixed element (generally a cylinder) is mounted on the engine block and a moveable element (generally a piston, moveable inside the cylinder) is slideable with respect to the fixed element and acts against the shoe placed in contact with the chain to tension it.
In these tensioning devices, the piston is biased out of the cylinder, towards the shoe disposed against a strand of the chain, by the action of a spring placed inside the cylinder.
When the tensioner is not installed, for example during transport and/or during possible replacement, the spring biases the piston (often with considerable force) out of the cylinder. To overcome this drawback, ways to lock the piston mechanically inside the cylinder at least during the transport of the tensioner have been developed (for example, a pin inserted in a through hole formed in the body of the piston and in a pair of holes made in the wall of the cylinder and aligned with the through hole formed in the piston).
This locking mechanism presents a serious drawback in that, when it is removed, the spring may violently expel the piston from the cylinder. This may make the installation of the tensioner difficult and may represent a source of danger for those carrying out the operation.
Furthermore, it is practically impossible, or at the very least, difficult and laborious, to re-install the locking mechanism before removing the tensioner for any reason including, but not limited to, carrying out the maintenance and/or the replacement of the tensioner and/or of the chain. The typically awkward position of the tensioner and the action of the spring on the piston may make this operation a source of danger for those carrying it out.